Development, Pregnancy, and Genetics Part 1 PDF

Development and Pregnancy Marie A. Román Martínez, PhD Department of Biology Office hours: by appointment Email: [email protected] Copyright-This presentation is intended for educational purpose only. No part of this presentation may be reproduced or transmitted in any form without written permiss...

Development and Pregnancy Marie A. Román Martínez, PhD Department of Biology Office hours: by appointment Email: [email protected] Copyright-This presentation is intended for educational purpose only. No part of this presentation may be reproduced or transmitted in any form without written permission. Objectives 1. Describe the processes of fertilization, pre-embryonic development, and implantation. 2. Explain the importance of the three germ layers. 3. Identify the extraembryonic membranes and their functions. 4. Describe the structure and function of the placenta. 5. Describe the fetal development. 6. Explain the fetal cardiovascular adaptations and their value to the fetus. 7. Describe the cardiovascular changes that occur in postnatal development. 8. Describe the control of lactation and milk ejection. 9. Describe disorders during pregnancy and the pre and postnatal development. 2 Introduction A new life begins as a single cell formed by the fusion of a sperm and an ovum. The first cell contains all of the inherited information it needs to grow and develop. Birth occurs after about 38 weeks of prenatal growth and development or 40 weeks from beginning of last menstruation. 3 Fertilization and Early Development A primary oocyte undergoes meiosis I to form a secondary oocyte and a polar body, each with 23 chromosomes. A secondary oocyte surrounded by granulosa cells is ovulated from the ovary and travels slowly towards the uterus. A mature (graafian) follicle. Just before ovulation, this follicle will grow to as much as 25 to 30 mm in diameter Figure 28.12 obtained from Saladin Anatomy & Physiology 9th edition Endoscopic Photo of a Human Ovulation from Figure 28.15 obtained from Saladin Anatomy & Physiology 9th edition 4 Fertilization Only a few thousand sperm reach the uterine tubes. Sperm swim against the fluid current flowing. Prostaglandins assist sperm movement by causing reverse peristalsis of the uterus and uterine tubes. Entire journey of a sperm to the uterine tubes takes approx. 1 hour. Usually only one uterine tube contains a secondary oocyte. Sperm are chemically attracted to the secondary oocyte. Cluster around it and attempt to penetrate the granulosa cells. 5 Fertilization Sperm use acrosomal enzymes to digest a path through the granulosa cells around the secondary oocyte. Once one sperm penetrates the oocyte, the oocyte plasma membrane changes. Blocks all other sperm from entering. Secondary oocyte completes meiosis II forming an ovum and a polar body. 6 Fertilization Fertilization occurs when the sperm and ovum nuclei fuse to form a zygote. Zygote is the first cell of the pre-embryo. Contains 46 chromosomes, 23 from the sperm and 23 from the ovum, and leads to the development of a human being. Fertilization Pre-embryo 2nd week of development Secondary oocytes are only viable for 24 hours after ovulation, while sperm are viable for about 72 hours. 7 Pre-embryo Development Zygote undergoes cleavage. Rapid division by mitosis– cell number increases without cell growth. When the pre-embryo reaches the uterus, it is called morula. Solid ball of cells not much larger than the zygote. 8 Pre-embryo Development Mitosis continues. The pre-embryo becomes a blastocyst. A larger, hollow ball of cells. The blastocyst has two parts: Embryoblast from which the embryo later develops. Trophoblast that helps to form the placenta. 9 Preembryo Development Implantation Blastocyst uses enzymes from the trophoblast to penetrate into the endometrium. Begins approx. day 7 after fertilization and completes by 14th day. 10 Germ Layers Embryonic discs forms from the embryoblast. It consists of three primary germ layers that form all body tissues and organs: 1. Ectoderm forms nervous system and epidermis. 2. Mesoderm middle layer, forms muscles, bone tissue, blood, and other connective tissues. 3. Endoderm forms the epithelial lining of digestive, respiratory, and urinary tracts. 11 Extraembryonic Membranes Chorion: Trophoblast becomes the outermost membrane. Chorionic villi: Extensions of trophoblast that have grown into endometrium. Amnion Forms posterior to the embryo and eventually surrounds it. Amniotic cavity, space between amnion and embryonic disc, is filled with amniotic fluid. Absorbs shocks. 12 Extraembryonic Membranes Yolk sac: Forms early formed elements and germ cells. Absorbs shocks and forms primitive gut. Allantois: Forms early formed elements. Brings umbilical blood vessels to placenta. Allantois and yolk sac form the umbilical cord. 13 Placenta Disc-shaped structure formed of both embryonic and maternal tissues. Embryonic portion: formed of the chorion and chorionic villi. Maternal portion: formed of the associated endometrium. Umbilical cord attaches the embryo to the placenta 2 umbilical arteries bring embryonic blood to placenta. 1 umbilical vein returns the blood to embryo. No nerves. 14 Placenta Placenta is the interface between the embryonic blood and maternal blood for exchange of materials: Water, respiratory gases, nutrients, wastes, hormones, and antibodies. Embryonic blood and maternal blood do not mix: Maternal blood vessels open into lacunae. Embryonic blood vessels extend into and branch in the lacunae. Diffusion is used to exchange material across embryonic blood vessels. 15 Embryonic Stage (3rd- 8th week) Offspring during this time is called an embryo. By the 4th week, head and limb buds are recognizable. By the 7th week, rudiments of all organs are present. Eyes and ears are visible. By the 8th week, the embryo has a distinct human appearance. 8th week embryo 16 Fetal Stage (9th- (38th -40th week) Developing offspring is now called a fetus. At start of fetal stage, head is as large as the body and all organs are present. 9th through 12th weeks: Bone ossification occurs and organs continue to develop. 13th through 16th weeks: Eyes and ears reach final positions. Heartbeat may be detected with a stethoscope. Image obtained from Figure 29.7 from Saladin Anatomy & Physiology 9th edition 17 Fetal Stage 17th through 20th weeks: Lanugo (fine hair) covers body and hair is on scalp. Vernix caseosa forms –protects the skin from the digestive and urinary wastes in the surrounding amniotic fluid. Movements may now be detectable by mother. By 20th week, weighs approx. 460 grams (1lb). 21st through 29th weeks: Head, body, and limbs reach infant proportions. Fetus is lean with wrinkled, translucent skin. By 29th week, weighs approx. 1300 grams (3 lbs). Image obtained from Figure 29.7 from Saladin Anatomy & Physiology 9th edition 18 Fetal Stage 30th weeks to birth: (8th and 9th months) Deposition of adipose tissue. Descent of testes in males. Lanugo is shed, the skin is pinkish with ample subcutaneous tissue, hair covers the scalp. Organ systems ready for birth. The fetus weighs about 3,400 grams (7.5 pounds). 19 Human chorionic gonadotropin (hCG) Produced by the trophoblast and chorion. Maintains the corpus luteum for the first 10 to 12 weeks of development: Continues to secrete high levels of progesterone and estrogens to maintain the endometrium. Detected by pregnancy tests within 8 to 10 days after fertilization. hCG concentration peaks after approx. 10 weeks of development. Blood hCG level declines and levels off between 16 and 20 weeks. Corpus luteum degenerates as a result. 20 Hormonal Control of Pregnancy Placenta By approx. 12 weeks, it takes over the role of producing estrogens and progesterone. Prevents the loss of the endometrium when the corpus luteum degenerates. High blood level of progesterone suppresses GnRH, which prevents the release of FSH and LH. The ovaries remain inactive during pregnancy. (a) The fetal side of the placenta after birth, showing blood vessels, the umbilical cord, and some of the amniotic sac attached to the lower left margin. (b) The maternal (uterine) side, where chorionic villi give the placenta a rougher texture. Image obtained from Figure 29.9 from Saladin Anatomy & Physiology 9 th edition 21 Hormonal Control Pregnancy Blood levels of both estrogens and progesterone increase during pregnancy. Stimulate mammary gland development. 22 Birth Progesterone inhibits uterine contractions during most of pregnancy. Blood concentration of estrogens increases as pregnancy nears full term. Estrogens promote uterine contractions. Braxton Hicks contractions “False labor”. Relaxin Secreted by corpus luteum and placenta. Helps in developing placental blood vessels and other maternal cardiovascular changes. 23 Birth Parturition (birth process): Usually occurs within 2 weeks of the calculated due date. Approx. 280 days from the beginning of the last menstruation. The fetus is usually in a “head down” position at the beginning of labor. 24 Labor First stage of labor: dilation of the cervix Longest stage: 6 to 12 hours Amnion ruptures as cervix dilates. Cervical stretching triggers formation of action potential that are sent to the hypothalamus. Hypothalamus activates the posterior lobe of the pituitary to release oxytocin. Oxytocin stimulates rhythmic contraction of the uterus. 25 Labor First stage of labor: Fetus head pushes against cervix, resulting in greater stretching. Causes more action potentials to be sent to the hypothalamus. Hypothalamus causes more oxytocin to be release from posterior lobe. Higher blood oxytocin level stimulates more intense and frequent uterine contractions. This positive-feedback mechanism produces increasingly stronger uterine contractions until birth occurs. Pitocin 26 Labor Second stage of labor: delivery of fetus. Lasts less than an hour. Contractions occur every 2 to 3 minutes, lasting about 1 minute. Once the head is expelled, the rest of the body exits rather quickly. 27 Labor Third stage of labor: delivery of the placenta. Occurs within 15 minutes of the birth of the infant. Caused by continued contractions of the uterus. Produces some bleeding because endometrial blood vessels at the placental site are ruptured. Uterine contractions compress broken blood vessels, which reduces bleeding. Subsequently, the uterus decreases in size rather quickly. 28 First Breath After birth, fluids are removed from infant’s nose and mouth. Umbilical cord is clamped and cut. Increase in blood CO2 level activates the respiratory rhythmicity center is activated. The first breath is difficult because the lungs are collapsed. Once first breath is performed, surfactant helps prevent alveolar collapse. Makes breathing easier. 29